For automatic placement of components on a circuit board, usually an assembly robot is used in a computer-controlled assembly robot cell provided with a circuit board conveyor, which brings the circuit board to an assembly station located within the working area of the robot. The assembly robot is preferably a xyzw-robot (also called as a xyzrz-robot) which refers to a robot having four degrees of freedom (x, y, z, w). The robot is provided with grippers for taking hold of a component. The robot is capable of three-dimensional motion in x, y and z directions which form the three degrees of freedom and the rotation of the grippers about z-axis forms the fourth degree of freedom (w). The robot is e.g. a portal robot working in the space above the circuit board conveyor. It fetches one component at a time from a component feed device and places it in a specified location on the circuit board. Before being placed on the circuit board, the component must be inspected to determine whether it is fit for assembly. The leads of the component must be aligned in a sufficiently straight line and they must not have too much sideways bends because in that case they would not go through the holes in the circuit board. It is also important to check the position of the leads of the component in relation to the gripper.
In the prior art, vision procedures and systems based on machine vision for inspecting electronic components to determine their fitness for assembly are known. This type of systems based on machine vision are manufactured e.g. by the American companies Adept Technology Inc. (San Jose, USA) and Cognex Corporation (Massachusetts, USA).
A prior-art system comprises an upward directed camera disposed at a fixed location beside a circuit board conveyor. A robot first takes each component, with the leads pointing downward, onto the optics of the camera, which takes a picture of the tips of the leads from below, whereupon a vision program comprised in the system computes the positions of the leads on the basis of the image and compares them with approved reference values. If the vision software finds that the detected lead positions are not within acceptable tolerances, i.e. one or more leads have been bent, then the software instructs the robot to reject the component and get a new one for inspection and assembly.
A problem with this prior-art procedure and system is that it is very expensive as it incorporates many components: a camera, lighting, a machine vision processor with software to develop applications, a video monitor to display images, user interface etc.
A further problem is that there are many factors impairing the reliability of machine vision. The image may be blurred due to reflections, and the contrast between the component body and the leads may be poor. Also, the colour of the component body visible on the background of the leads may cause blurring of the image. The leads may not be distinct enough from the background. In order to be able to distinguish the leads from the background of the component bodies of different colours one would need to arrange different kind of illuminations specific for each colour.
In principle, this problem could be solved in the machine vision system by taking two pictures of the component from its two sides from one end of the lead row instead of photographing it from below. Such a solution is applicable in the case of small components that fit completely in the image area of the camera. However, the problem associated with the machine vision system become more pronounced especially in the case of large components that do not fit in the image area of the camera. To achieve a reliable and sufficiently accurate inspection result, it is necessary to use macro optics and a small image area. For instance, a large connector, which may have as many as hundreds of leads arranged in straight rows in two directions perpendicular to each other, will not fit in the image area at once, so that, in order to produce a picture of the entire row of leads, the camera would have to be moved in relation to the component or vice versa during the inspection to produce two or more pictures of the same component. A complicated camera scanning method like this, intended for surface mounted components, is described in specification U.S. Pat. No. 5,805,722.
In the case of large components with long rows of leads, the long row of leads cannot be reliably inspected using a camera even if pictures are taken from the end of the row. The reason for this is the poor depth resolution ad parallox error of the required camera optics.
There are also systems in which a picture of the component is taken in motion, but this imposes great demands on the vision software, which therefore becomes heavy and expensive.
A further problem is that taking the component aside from the route between the component feed device and the circuit board in order to take a picture of its significantly retards the assembly process.
The object of the present invention is to eliminate the drawbacks described below.
A specific object of the present invention is to disclose a simple, cheap and reliable inspection procedure and system for inspecting a component to determine its fitness for assembly without substantially retarding the assembly process.
A further object of the invention is to disclose a procedure and system that allow a component fitness inspection to be performed while the component is being transferred to an assembly station, without stopping the transferring motion.
An additional object of the invention is to disclose a procedure and system by which it is possible to reliably inspect the leads of components and ascertain that the leads have not been bent, on all components regardless of the number of leads (e.g. 2-500) and the colour and lead profile of the component.
As for the features characteristic of the invention, reference is made to the claims.